1. Field of the Invention
This invention relates to a data transfer system using a memory with FIFO (first-in first-out) structure, and more particularly to a multiplex communication system for use in a car, for example.
2. Description of the Related Art
In a multiplex communication system for use in a car, ID codes for identifying various data items such as the rotation speed of the engine, the speed of the car, the rotation speed of the wheel and the temperature of coolant are added to the respective data items and supplied to a network line, various control units for controlling the engine, brake and the like are connected to the network line. Each of the control units includes an ID code comparing/processing section. Each of the ID code comparing/processing sections includes multiplex communication control units of the same number as the ID codes and an OR gate for receiving coincidence signals output from the multiplex communication control units and informing a CPU of a fact that data necessary for the control unit. The multiplex communication control unit includes an ID register and a comparator for comparing ID codes previously stored in the ID register with an ID code received from the network line. Data corresponding to the coincident ID code is transferred to the CPU to control the engine, brake and the like to set them into optimum conditions. This type of multi-plex communication system for use in a car is disclosed as the prior art in Japanese Patent Disclosure No. 3-250830, for example.
However, with the system disclosed as the prior art in the above Japanese Patent Disclosure, it is required for the CPU to access the multiplex communication control units by as many times as the number of types of data items occurring in the worst case in order to determine which one of the plurality of ID codes is received from the network line by accessing each of the multiplex communication control units. Therefore, as the number of types of data items increases, it takes a longer time to identify the received ID code and transfer the received data to the CPU. This is particularly disadvantageous in a system for use on a car in which data transfer on the real time basis is required and the load of the CPU becomes heavy.
Further, since the time-series processing cannot be effected in the above conventional system, an ID code which has been received before may be lost when a plurality of ID codes are successively received from the network line, that is, when a succeeding ID code is received while the CPU accesses the multiplex communication control unit and identifies the ID code, and data processing for the lost ID code cannot be effected.